Basic File Attributes

Transcription

1 Basic File Attributes The UNIX file system allows the user to access other files not belonging to them and without infringing on security. A file has a number of attributes (properties) that are stored in the inode. In this chapter, we discuss, ls l to display file attributes (properties) Listing of a specific directory Ownership and group ownership Different file permissions Listing File Attributes ls command is used to obtain a list of all filenames in the current directory. The output in UNIX lingo is often referred to as the listing. Sometimes we combine this option with other options for displaying other attributes, or ordering the list in a different sequence. ls look up the file s inode to fetch its attributes. It lists seven attributes of all files in the current directory and they are: File type and Permissions Links Ownership Group ownership File size Last Modification date and time File name The file type and its permissions are associated with each file. Links indicate the number of file names maintained by the system. This does not mean that there are so many copies of the file. File is created by the owner. Every user is attached to a group owner. File size in bytes is displayed. Last modification time is the next field. If you change only the permissions or ownership of the file, the modification time remains unchanged. In the last field, it displays the file name. For example, $ ls l total 72 -rw-r--r-- 1 kumar metal may 10 13:45 chap01 -rw-r--r-- 1 kumar metal 4174 may 10 15:01 chap02 -rw-rw-rw- 1 kumar metal 84 feb 12 12:30 dept.lst -rw-r--r-- 1 kumar metal 9156 mar genie.sh drwxr-xr-x 2 kumar metal 512 may 9 10:31 helpdir

2 drwxr-xr-x 2 kumar metal 512 may 9 09:57 progs Listing Directory Attributes ls -d will not list all subdirectories in the current directory For example, ls ld helpdir progs drwxr-xr-x 2 kumar metal drwxr-xr-x 2 kumar metal 512 may 9 10:31 helpdir 512 may 9 09:57 progs Directories are easily identified in the listing by the first character of the first column, which here shows a d. The significance of the attributes of a directory differs a good deal from an ordinary file. To see the attributes of a directory rather than the files contained in it, use ls ld with the directory name. Note that simply using ls d will not list all subdirectories in the current directory. Strange though it may seem, ls has no option to list only directories. File Ownership When you create a file, you become its owner. Every owner is attached to a group owner. Several users may belong to a single group, but the privileges of the group are set by the owner of the file and not by the group members. When the system administrator creates a user account, he has to assign these parameters to the user: The user-id (UID) both its name and numeric representation The group-id (GID) both its name and numeric representation File Permissions UNIX follows a three-tiered file protection system that determines a file s access rights. It is displayed in the following format: For Example: Filetype owner (rwx) groupowner (rwx) others (rwx) -rwxr-xr-- 1 kumar metal may 10 19:21 chap02 r w x r - x r - - owner/user group owner others The first group has all three permissions. The file is readable, writable and executable by the owner of the file. The second group has a hyphen in the middle slot,

3 which indicates the absence of write permission by the group owner of the file. The third group has the write and execute bits absent. This set of permissions is applicable to others. You can set different permissions for the three categories of users owner, group and others. It s important that you understand them because a little learning here can be a dangerous thing. Faulty file permission is a sure recipe for disaster Changing File Permissions A file or a directory is created with a default set of permissions, which can be determined by umask. Let us assume that the file permission for the created file is -rw-r--r--. Using chmod command, we can change the file permissions and allow the owner to execute his file. The command can be used in two ways: In a relative manner by specifying the changes to the current permissions In an absolute manner by specifying the final permissions Relative Permissions chmod only changes the permissions specified in the command line and leaves the other permissions unchanged. Its syntax is: chmod category operation permission filename(s) chmod takes an expression as its argument which contains: user category (user, group, others) operation to be performed (assign or remove a permission) type of permission (read, write, execute) Category operation permission u - user + assign r - read g - group - remove w - write o - others = absolute x - execute a - all (ugo) Let us discuss some examples: Initially, -rw-r--r-- 1 kumar metal 1906 sep 23:38 xstart chmod u+x xstart -rwxr--r-- 1 kumar metal 1906 sep 23:38 xstart

4 The command assigns (+) execute (x) permission to the user (u), other permissions remain unchanged. chmod ugo+x xstart chmod a+x xstart chmod +x xstart or or -rwxr-xr-x 1 kumar metal 1906 sep 23:38 xstart chmod accepts multiple file names in command line chmod u+x note note1 note3 Let initially, -rwxr-xr-x 1 kumar metal 1906 sep 23:38 xstart chmod go-r xstart Then, it becomes -rwx--x--x 1 kumar metal 1906 sep 23:38 xstart Absolute Permissions Here, we need not to know the current file permissions. We can set all nine permissions explicitly. A string of three octal digits is used as an expression. The permission can be represented by one octal digit for each category. For each category, we add octal digits. If we represent the permissions of each category by one octal digit, this is how the permission can be represented: Read permission 4 (octal 100) Write permission 2 (octal 010) Execute permission 1 (octal 001) Octal Permissions Significance no permissions x execute only 2 - w - write only 3 - w x write and execute 4 r - - read only 5 r - x read and execute 6 r w - read and write 7 r w x read, write and execute

5 We have three categories and three permissions for each category, so three octal digits can describe a file s permissions completely. The most significant digit represents user and the least one represents others. chmod can use this three-digit string as the expression. Using relative permission, we have, chmod a+rw xstart Using absolute permission, we have, chmod 666 xstart chmod 644 xstart chmod 761 xstart will assign all permissions to the owner, read and write permissions for the group and only execute permission to the others. 777 signify all permissions for all categories, but still we can prevent a file from being deleted. 000 signifies absence of all permissions for all categories, but still we can delete a file. It is the directory permissions that determine whether a file can be deleted or not. Only owner can change the file permissions. User can not change other user s file s permissions. But the system administrator can do anything. The Security Implications Let the default permission for the file xstart is -rw-r--r-- chmod u-rw, go-r xstart or chmod 000 xstart This is simply useless but still the user can delete this file On the other hand, chmod a+rwx xstart chmod 777 xstart -rwxrwxrwx

6 The UNIX system by default, never allows this situation as you can never have a secure system. Hence, directory permissions also play a very vital role here We can use chmod Recursively. chmod -R a+x shell_scripts This makes all the files and subdirectories found in the shell_scripts directory, executable by all users. When you know the shell meta characters well, you will appreciate that the * doesn t match filenames beginning with a dot. The dot is generally a safer but note that both commands change the permissions of directories also. Directory Permissions It is possible that a file cannot be accessed even though it has read permission, and can be removed even when it is write protected. The default permissions of a directory are, rwxr-xr-x (755) A directory must never be writable by group and others Example: mkdir c_progs ls ld c_progs drwxr-xr-x 2 kumar metal 512 may 9 09:57 c_progs If a directory has write permission for group and others also, be assured that every user can remove every file in the directory. As a rule, you must not make directories universally writable unless you have definite reasons to do so. Changing File Ownership Usually, on BSD and AT&T systems, there are two commands meant to change the ownership of a file or directory. Let kumar be the owner and metal be the group owner. If sharma copies a file of kumar, then sharma will become its owner and he can manipulate the attributes chown changing file owner and chgrp changing group owner On BSD, only system administrator can use chown On other systems, only the owner can change both

7 chown Changing ownership requires superuser permission, so use su command ls -l note -rwxr----x 1 kumar metal 347 may 10 20:30 note chown sharma note; ls -l note -rwxr----x 1 sharma metal 347 may 10 20:30 note Once ownership of the file has been given away to sharma, the user file permissions that previously applied to Kumar now apply to sharma. Thus, Kumar can no longer edit note since there is no write privilege for group and others. He can not get back the ownership either. But he can copy the file to his own directory, in which case he becomes the owner of the copy. chgrp This command changes the file s group owner. No superuser permission is required. ls l dept.lst -rw-r--r-- 1 kumar metal 139 jun 8 16:43 dept.lst chgrp dba dept.lst; ls l dept.lst -rw-r--r-- 1 kumar dba 139 jun 8 16:43 dept.lst In this chapter we considered two important file attributes permissions and ownership. After we complete the first round of discussions related to files, we will take up the other file attributes.

8 The vi Editor To write and edit some programs and scripts, we require editors. UNIX provides vi editor for BSD system created by Bill Joy. Bram Moolenaar improved vi editor and called it as vim (vi improved) on Linux OS. vi Basics To add some text to a file, we invoke, vi <filename> In all probability, the file doesn t exist, and vi presents you a full screen with the filename shown at the bottom with the qualifier. The cursor is positioned at the top and all remaining lines of the screen show a ~. They are non-existent lines. The last line is reserved for commands that you can enter to act on text. This line is also used by the system to display messages. This is the command mode. This is the mode where you can pass commands to act on text, using most of the keys of the keyboard. This is the default mode of the editor where every key pressed is interpreted as a command to run on text. You will have to be in this mode to copy and delete text For, text editing, vi uses 24 out of 25 lines that are normally available in the terminal. To enter text, you must switch to the input mode. First press the key i, and you are in this mode ready to input text. Subsequent key depressions will then show up on the screen as text input. After text entry is complete, the cursor is positioned on the last character of the last line. This is known as current line and the character where the cursor is stationed is the current cursor position. This mode is used to handle files and perform substitution. After the command is run, you are back to the default command mode. If a word has been misspelled, use ctrl-w to erase the entire word. Now press esc key to revert to command mode. Press it again and you will hear a beep. A beep in vi indicates that a key has been pressed unnecessarily. Actually, the text entered has not been saved on disk but exists in some temporary storage called a buffer. To save the entered text, you must switch to the execute mode (the last line mode). Invoke the execute mode from the command mode by entering a: which shows up in the last line. The Repeat Factor vi provides repeat factor in command and input mode commands. Command mode command k moves the cursor one line up. 10k moves cursor 10 lines up. To undo whenever you make a mistake, press Esc u

9 To clear the screen in command mode, press ctrl-l Don t use (caps lock) - vi commands are case-sensitive Avoid using the PC navigation keys Input Mode Entering and Replacing Text It is possible to display the mode in which is user is in by typing, :set showmode Messages like INSERT MODE, REPLACE MODE, CHANGE MODE, etc will appear in the last line. Pressing i changes the mode from command to input mode. To append text to the right of the cursor position, we use a, text. I and A behave same as i and a, but at line extremes I inserts text at the beginning of line. A appends text at end of line. o opens a new line below the current line r<letter> replacing a single character s<text/word> replacing text with s R<text/word> replacing text with R Press esc key to switch to command mode after you have keyed in text Some of the input mode commands are: COMMAND FUNCTION i inserts text a appends text I inserts at beginning of line A appends text at end of line o opens line below O opens line above r replaces a single character s replaces with a text S replaces entire line Saving Text and Quitting The ex Mode When you edit a file using vi, the original file is not distributed as such, but only a copy of it that is placed in a buffer. From time to time, you should save your work by writing the buffer contents to disk to keep the disk file current. When we talk of saving a

10 file, we actually mean saving this buffer. You may also need to quit vi after or without saving the buffer. Some of the save and exit commands of the ex mode is: Navigation Command Action :W saves file and remains in editing mode :x saves and quits editing mode :wq saves and quits editing mode :w <filename> save as :w! <filename> save as, but overwrites existing file :q quits editing mode :q! quits editing mode by rejecting changes made :sh escapes to UNIX shell :recover recovers file from a crash A command mode command doesn t show up on screen but simply performs a function. To move the cursor in four directions, k j h l moves cursor up moves cursor down moves cursor left moves cursor right Word Navigation Moving by one character is not always enough. You will often need to move faster along a line. vi understands a word as a navigation unit which can be defined in two ways, depending on the key pressed. If your cursor is a number of words away from your desired position, you can use the word-navigation commands to go there directly. There are three basic commands: b e w moves back to beginning of word moves forward to end of word moves forward to beginning word Example, 5b takes the cursor 5 words back 3w takes the cursor 3 words forward Moving to Line Extremes Moving to the beginning or end of a line is a common requirement.

11 To move to the first character of a line 0 or 30 moves cursor to column 30 $ moves to the end of the current line The use of these commands along with b, e, and w is allowed Scrolling Faster movement can be achieved by scrolling text in the window using the control keys. The two commands for scrolling a page at a time are ctrl-f ctrl-b scrolls forward scrolls backward 10ctrl-f scroll 10 pages and navigate faster ctrl-d ctrl-u scrolls half page forward scrolls half page backward The repeat factor can also be used here. Absolute Movement The editor displays the total number of lines in the last line Ctrl-g to know the current line number 40G goes to line number 40 1G goes to line number 1 G goes to end of file Editing Text The editing facilitates in vi are very elaborate and invoke the use of operators. They use operators, such as, d y delete yank (copy) Deleting Text x dd yy deletes a single character delete entire line copy entire line

12 6dd deletes the current line and five lines below Moving Text Moving text (p) puts the text at the new location. p and P place text on right and left only when you delete parts of lines. But the same keys get associated with below and above when you delete complete lines Copying Text Copying text (y and p) is achieved as, yy 10yy copies current line copies current line & 9 lines below Joining Lines J 4J to join the current line and the line following it joins following 3 lines with current line Undoing Last Editing Instructions In command mode, to undo the last change made, we use To discard all changes made to the current line, we use u U vim (LINUX) lets you undo and redo multiple editing instructions. u behaves differently here; repeated use of this key progressively undoes your previous actions. You could even have the original file in front of you. Further 10u reverses your last 10 editing actions. The function of U remains the same. You may overshoot the desired mark when you keep u pressed, in which case use ctrl-r to redo your undone actions. Further, undoing with 10u can be completely reversed with 10ctrl-r. The undoing limit is set by the execute mode command: set undolevels=n, where n is set to 1000 by default. Repeating the Last Command The. (dot) command is used for repeating the last instruction in both editing and command mode commands For example: 2dd deletes 2 lines from current line and to repeat this operation, type. (dot) Searching for a Pattern

13 / search forward? search backward /printf The search begins forward to position the cursor on the first instance of the word?pattern Searches backward for the most previous instance of the pattern Repeating the Last Pattern Search n repeats search in same direction of original search n doesn t necessarily repeat a search in the forward direction. The direction depends on the search command used. If you used? printf to search in the reverse direction in the first place, then n also follows the same direction. In that case, N will repeat the search in the forward direction, and not n. Search and repeat commands Command Function /pat?pat n N searches forward for pattern pat searches backward for pattern pat repeats search in same direction along which previous search was made repeats search in direction opposite to that along which previous search was made Substitution search and replace We can perform search and replace in execute mode using :s. Its syntax is, :address/source_pattern/target_pattern/flags :1,$s/director/member/g can also use % instead of 1,$ :1,50s/unsigned//g deletes unsigned everywhere in lines 1 to 50 :3,10s/director/member/g substitute lines 3 through 10 :.s/director/member/g only the current line :$s/director/member/g only the last line Interactive substitution: sometimes you may like to selectively replace a string. In that case, add the c parameter as the flag at the end: :1,$s/director/member/gc

14 Each line is selected in turn, followed by a sequence of carets in the next line, just below the pattern that requires substitution. The cursor is positioned at the end of this caret sequence, waiting for your response. The ex mode is also used for substitution. Both search and replace operations also use regular expressions for matching multiple patterns. The features of vi editor that have been highlighted so far are good enough for a beginner who should not proceed any further before mastering most of them. There are many more functions that make vi a very powerful editor. Can you copy three words or even the entire file using simple keystrokes? Can you copy or move multiple sections of text from one file to another in a single file switch? How do you compile your C and Java programs without leaving the editor? vi can do all this. Source: Sumitabha Das, UNIX Concepts and Applications, 4 th edition, Tata McGraw Hill, 2006